1. Field of the Invention
The present invention relates to a color converting apparatus and color converting method for converting input image data into converted image data for a color outputting device.
2. Description of Related Art
In order to reproduce a color by using a color outputting device, such as a printer, equivalent to that indicated by input image data, a color matching process is employed in the process of converting the input image data into control signals for the color outputting device. During the conversion process, in order to attain a color matching, the input image data is converted to device-independent XYZ color quantities (X, Y, Z), and then into Lab color quantities (L*, a*, b*), which are also device-independent.
Conventionally, when an arbitrary object color is produced by a color outputting device, such as a printer, using paper, ink, or another color reproducing media, a set of XYZ color values (X, Y, Z) is defined as indicative of the arbitrary color by the following Equation (1):
                                                        X              =                              K                *                                                      ∫                    880                    780                                    ⁢                                                            (                                                                        S                          ⁡                                                      (                            λ                            )                                                                          *                                                  B                          ⁡                                                      (                            λ                            )                                                                          *                                                  x                          ⁡                                                      (                            λ                            )                                                                                              )                                        ⁢                                                                                  ⁢                                          ⅆ                      λ                                                                                                                                              Y              =                              K                *                                                      ∫                    880                    780                                    ⁢                                                            (                                                                        S                          ⁡                                                      (                            λ                            )                                                                          *                                                  B                          ⁡                                                      (                            λ                            )                                                                          *                                                  y                          ⁡                                                      (                            λ                            )                                                                                              )                                        ⁢                                                                                  ⁢                                          ⅆ                      λ                                                                                                                                              Z              =                              K                *                                                      ∫                    880                    780                                    ⁢                                                            (                                                                        S                          ⁡                                                      (                            λ                            )                                                                          *                                                  B                          ⁡                                                      (                            λ                            )                                                                          *                                                  z                          ⁡                                                      (                            λ                            )                                                                                              )                                        ⁢                                                                                  ⁢                                          ⅆ                      λ                                                                                                                                              K              =                              100                /                                                      ∫                    880                    780                                    ⁢                                                            (                                                                        S                          ⁡                                                      (                            λ                            )                                                                          *                                                  y                          ⁡                                                      (                            λ                            )                                                                                              )                                        ⁢                                                                                  ⁢                                          ⅆ                      λ                                                                                                                              (        1        )                            wherein * denotes a multiplication symbol; S(λ) denotes a relative spectral power distribution of light with wavelength λ incident on the object color; B(λ) denotes the spectral reflectance characteristics of the object color for the light with wavelength λ; x(λ), y(λ), and z(λ) are three sensitivities of the human eye for the wavelength λ. These three sensitivities x(λ), y(λ), and z(λ) are greater than zero (0) only within the visible range of about 380-700 nm.        
A set of Lab color quantities (L*, a*, b*) is defined in terms of the XYZ value set (X, Y, Z) by the following Equation (2).L*=116*(Y/Yn)1/3−16a*=500*{(X/Xn)1/3−(Y/Yn)1/3)}b*=200*{(Y/Yn)1/3−(Z/Zn)1/3)}  (2)                wherein Xn, Yn, and Zn are tristimulus values of a perfectly diffuse surface (white). In other words, Xn, Yn, and Zn are tristimulus values defined for the case where incident light is illuminated directly on the human eye. These tristimulus values Xn, Yn, and Zn are defined by Equation (3) below:        
                                                        Xn              =                              K                *                                                      ∫                    880                    780                                    ⁢                                                            (                                                                        S                          ⁡                                                      (                            λ                            )                                                                          *                                                  x                          ⁡                                                      (                            λ                            )                                                                                              )                                        ⁢                                                                                  ⁢                                          ⅆ                      λ                                                                                                                                              Yn              =                              K                *                                                      ∫                    880                    780                                    ⁢                                                            (                                                                        S                          ⁡                                                      (                            λ                            )                                                                          *                                                  y                          ⁡                                                      (                            λ                            )                                                                                              )                                        ⁢                                                                                  ⁢                                          ⅆ                      λ                                                                                                                                              Zn              =                              K                *                                                      ∫                    880                    780                                    ⁢                                                            (                                                                        S                          ⁡                                                      (                            λ                            )                                                                          *                                                  z                          ⁡                                                      (                            λ                            )                                                                                              )                                        ⁢                                                                                  ⁢                                          ⅆ                      λ                                                                                                                              (        3        )            
In recent years, inkjet printers have become widely used as the color outputting device.